In order to realize a semiconductor laser having wavelength stability, there may be exemplified a DBR laser and a DFB laser in which a grating is monolithically formed in the semiconductor laser, and an external cavity laser in which a fiber grating (FBG) is attached to the outside of the laser. These lasers are based on the principle that a portion of the laser light is returned to the laser by using a mirror having the wavelength selectivity utilizing the Bragg reflection to achieve the stable wavelength operation.
The DBR laser achieves a resonator by forming concave and convex portions at a waveguide surface on the extension of a waveguide of an active layer by the Bragg reflection. Since this laser is provided with diffraction gratings on the both ends of an optical waveguide layer, a light emitted from the active layer propagates through the optical waveguide layer, whereby a portion thereof is reflected by this diffraction grating and is then returned to a current injection part to be amplified. Since only a light having a specific wavelength is reflected in a predetermined direction from the diffraction grating, the wavelength of the laser light becomes constant.
Moreover, as this application, there have been developed an external cavity type semiconductor laser in which the diffraction grating is a component different from the semiconductor and a resonator is formed outside the semiconductor. The type of the laser results in a laser excellent in the wavelength stability, the temperature stability and the controllability. As the external resonator, there are fiber Bragg gratings (FBG) and a volume hologram gratings (VHG). Since the diffraction grating is constituted as a member different from the semiconductor laser, there is such a feature that the reflectivity and the resonator length can be individually designed. Since it is not subjected to the influence of temperature elevation by heat generation due to current injection, the wavelength stability can be further improved. In addition, the diffraction grating may be designed in conjunction with the resonator length, because the change in the refractive index of the semiconductor depending on the temperature is different from that of the diffraction grating, thereby making it possible to enhance the temperature stability.
Patent Document 1 (Japanese Patent Publication No. 2002-134833A) discloses an external cavity laser utilizing a grating formed at a quartz glass waveguide. This aims at providing a laser with the frequency stability, which can be used, without a temperature controller, under an environment where the room temperature greatly changes (e.g. 30° C. or higher). In addition, it describes that there is provided a temperature independent laser in which mode hopping is suppressed and there is no temperature dependency of the oscillation frequency.
It has been examined to use a nanoimprinting method as a method for forming a diffraction grating possessed by a semiconductor laser device. Adoption of the nanoimprinting method for forming the diffraction grating has an advantage that manufacturing costs of devices such as semiconductor laser and so forth can be reduced.
When forming a diffraction grating by nanoimprinting method, a resin layer is first formed on a semiconductor layer to form the diffraction grating. Then, a mold possessing the pattern of grooves and bumps corresponding to shape of this diffraction grating is pressed to this resin layer, and the resin layer is cured in that state. In this manner, the pattern of grooves and bumps of the mold is transferred to the resin layer. Then, a microstructure is formed in a semiconductor layer by transferring the shape of this resin layer to the semiconductor layer.
It has been disclosed in NON-PATENT DOCUMENT 1 that a nanoimprint technology is applied to prepare optical devices. A wavelength selective element, a reflection controlling element, a moth-eye structure and so forth are exemplified as such optical devices.